Zeolites are crystalline or quasi-crystalline aluminosilicates constructed of repeating TO4 tetrahedral units with T being most commonly Si, Al or P (or combinations of tetrahedral units). These units are linked together to form frameworks having regular cavities and/or channels of molecular dimensions within the crystal. Numerous types of synthetic zeolites have been synthesized and each has a unique framework based on the specific arrangement its tetrahedral units. By convention, each topological type is assigned a unique three-letter code (e.g., “AEI”) by the International Zeolite Association (IZA).
Zeolites have numerous industrial applications, and zeolites of certain frameworks, such as AEI, are known to be effective catalyst for treating combustion exhaust gas in industrial applications including internal combustion engines, gas turbines, coal-fired power plants, and the like. In one example, nitrogen oxides (NOx) in the exhaust gas may be controlled through a so-called selective catalytic reduction (SCR) process whereby NOx compounds in the exhaust gas are contacted with a reducing agent in the presence of a zeolite catalyst. Other industrial applications include the use of zeolites for methanol-to-olefin (MTO) applications.
Synthetic zeolites of the AEI topological type when prepared as aluminosilicate compositions are produced using structure directing agents (SDAs), also referred to as a “templates” or “templating agents”. The SDAs that are used in the preparation of aluminosilicate AEI topological type materials are typically complex organic molecules which guide or direct the molecular shape and pattern of the zeolite's framework. Generally, the SDA can be considered as a mold around which the zeolite crystals form. After the crystals are formed, the SDA is removed from the interior structure of the crystals, leaving a molecularly porous aluminosilicate cage.
In typical synthesis techniques, solid zeolite crystals are formed from a reaction mixture which contains the framework reactants (e.g., a source of silica and a source of alumina), a source of hydroxide ions (e.g., NaOH), and an SDA. Such synthesis techniques usually take several days (depending on factors such as crystallization temperature) to achieve the desired crystallization. When crystallization is complete, the solid product containing the zeolite crystals is separated from the mother liquor which is discarded. This discarded mother liquor contains unused SDA, which is often degraded due to harsh reaction conditions, and unreacted silica.
Known SDAs for use in AEI zeolite synthesis are relatively expensive and contribute to a substantial portion of the cost of manufacturing the zeolite. In addition, conventional methods for synthesizing zeolite AEI have a relatively poor yield based on the SDA (a key component of the reaction mixture) which also impacts manufacturing costs.
U.S. Pat. No. 5,958,370 describes conditions for the synthesis of SSZ-39, also known as AEI. Sodium silicate was used as the source of silica, while one of the following zeolites was used as an alumina source: NH4Y (SAR˜5.2), NaY (SAR˜5.2), dealuminated USY (SAR˜13). Sodium hydroxide and sodium silicate were used as bases while various templates were used (see Table 1.2). Table A of U.S. Pat. No. 5,958,370 shows the typical and preferred ratios of the components in the reaction mixture.
TABLE AReaction MixtureGenericTypicalPreferredYO2/WaOb 10-10015-60OH—/YO20.5-1.00.6-0.8Q/YO20.05-0.500.10-0.20M2/n/YO20.30-1.0 0.50-0.60H2O/YO220-8030-40Y, W, Q, M and n are defined. When Y is silicon; W is aluminum, Q is a SDA, M is a sodium cation and n=1. The examples used extremely small batch sizes, with one example producing 2.1 g of product and the others producing less than 0.5 g of product.
Moliner, et al. (Chem. Commun., 48 (2012) 8264) described the synthesis of Cu-exchanged SSZ-39 using sodium silicate or LUDOX as a silicon source, USY_CBV500, aluminum hydroxide or alumina, as the aluminum source and N—N-dimethyl-3,5-dimethylpiperidinium cation (Template H in U.S. Pat. No. 5,958,370) as the SDA. Of the twelve different reaction conditions tried, only three resulted in the formation of AEI after crystallization at 135° C. for 7 days under static conditions. Information on the size of the reaction mixture used was not provided. Only the use of NH4Y, but not alumina or aluminum hydroxide, as the alumina source resulted in the formation of AEI. Only the use of sodium silicate, but not silica sol, as the silica source resulted in the formation of AEI. More dilute reaction mixture favored the formation of AEI.
Accordingly, it would be desirable to reduce the cost of the synthesis process, preferably by a means that has a low impact on the environment. This invention satisfies this need amongst others.